1. Field of the Invention
The present invention relates to a memory array of magnetic storage cells, and more particularly to a monitor for detecting magnetic field events.
2. Discussion of Related Art
Magnetic random access memory (MRAM) technology utilizes storage cells. The storage cells or MTJ (Magnetic Tunnel Junction) each have at least two magnetic regions or layers with an electrically insulating barrier layer between them. The storage mechanism relies on the relative orientation of the magnetization of the two layers, and on the ability to discern this orientation by electrical means through electrodes attached to these layers.
MRAM memory arrays include an array of MTJs positioned at the intersections of wordlines and bitlines. Each MTJ includes a magnetically changeable (e.g., reversible) or free region, and a proximate magnetic reference or fixed region, arranged into an MTJ device. Reference region denotes any type of region which, in cooperation with the free or changeable region, results in a detectable state of the device as a whole.
The principle underlying storage of data in such cells is the ability to change, and even reverse, the relative orientation of the magnetization of the free and reference regions by changing the direction of magnetization along the easy axis (EA) of the free region, and the ability to thereafter read the relative orientation difference.
More particularly, data can be written to an MRAM cell by reversing the free region magnetization using applied bi-directional electrical and resultant magnetic stimuli via its respective bitline and wordline.
The MRAM cell is later read by measuring the resultant tunneling resistance of the device, which can assume different values depending on the relative orientation of the magnetization of the free region with respect to the reference region. If the free region is modeled as a simple elemental magnet having a direction of magnetization that is free to rotate but with a strong preference for aligning in either direction along its easy axis (+EA or −EA), and if the reference region is a similar elemental magnet but having a direction of magnetization fixed in the +EA direction, then two states, and therefore two possible tunneling resistance values, are defined for the cell: aligned (+EA/+EA) and anti-aligned (−EA/+EA).
As an example, and with reference to FIGS. 1a and 1b, in an MRAM array, magnetic memory cells are positioned at the intersections of an exemplary rectangular grid of electrically conductive lines 1-6. The lines are arranged over a substrate and cross, thereby forming intersecting regions at which the magnetic memory cells are positioned, e.g., magnetic memory cell 7. The MTJ 8 comprises at least two magnetic regions or layers with an electrically insulating barrier between them. The storage mechanism relies on the relative orientation of the magnetization of the two layers, and on the ability to discern the orientation through electrodes attached to the layers.
The MTJ 8 comprises a free magnetic region 9, a tunneling barrier layer 10, and a reference magnetic region 11. The ability of this type of storage cell to store electrically accessible data hinges on electron tunneling between these two regions, which in turn is dependent on the relative directions of magnetization of these two regions. Rotating the magnetization in the free region into one of two selectable directions in a bi-stable manner results in binary state stored in the storage cell. If the storage cell is oriented with its magnetic EA disposed horizontally then an electrical writing current flowing through a vertical line will apply an EA magnetic field to the cell, and a current flowing through a horizontal line will apply a hard-axis (HA) magnetic field to the cell.
In one implementation of MRAM cells, the writing of individual storage cells adheres to a concept referred to as an astroid for switching. The switching threshold of a single free region depends on a combination of EA and HA magnetic fields applied thereto. The Stoner-Wohlfarth astroid model, shown in FIG. 2, illustrates these threshold values in the plane of applied EA and HA fields. Switching occurs when the combination of EA and HA fields at the storage cell results in a vector outside of the astroid. Vectors inside the astroid will not switch the storage cell from a current bi-stable state. The astroid model also illustrates how the EA field needed to switch a device is reduced in the presence of an HA bias field. Selectively switching a single storage cell within the array is achieved by applying electrical currents through a selected pair of horizontal and vertical lines. These currents generate a combination of EA and HA fields only at the storage cell located at the intersection of these lines, switching the selected cell, but not the neighboring cells.
All the storage cells along the selected horizontal line will experience the same applied HA field. Similarly all the storage cells along the selected vertical line will experience the same applied EA field. However, only the storage cell at the intersection of these lines will experience the combination of both fields needed for switching.
While MRAM has demonstrated the capacity for long term retention of information, as with many magnetic storage devices, such as disk drives, or tape, the data stored on the device can be erased, accidentally or purposefully, by exposure to a sufficiently large magnetic field.
Therefore, a need exists for a system and method of monitoring field events that can potentially erase data stored on an MRAM device.